Plant Kingdom of Class 11
Water is indispensable for fertilization (zooidogamy).
Many antherozoids swim to the neck of archegonium.
All the neck canal cells and venter canal cells disorganise to form mucilage, carbohydrate, proteins, K+, etc. These chemicals not only provide the medium for swimming of antherozoids but also are chemotaetic in nature. Many antherozoids enter into the venter but only one, the most active one, fuses with egg to form diploid zygote (oospore). With the formation of diploid zygote, the gametophytic generation ends and the sporophytic generation starts.
The zygote, immediately after fertilization, divides repeatedly without undergoing any resting period. The first division of zygote is transverse and the embryo proper develops from the outer cell. This type of embyrogeny is commonly known as exoscopic embryogeny.
The embryo is not liberated but is retained within the archegonium where it develops into a sporophyte. The sporophyte in bryophytes is called sporogonium because it is dependent and mainly meant for producing spores.
The sporophyte consists of foot, seta and capsule. In a few cases only seta is absent as in Corsinia whereas in Riccia both foot and seta are absent. Wall of the venter proliferates to form a covering around the growing embryo. It is called calyptra.
In a capsule, the spores are formed after meiosis. These spores (meiospores), that are all of one kind make the plants homosporous.
The Young Gametophyte
The spore is the mother (first) cell of the gametophytic generation.
The spores are cutinized and non motile. They are exclusively wind disseminated. The individual spores separate from the tetrad before they are discharged from the capsule.
The spores germinate directly into the new gametophytic plants (e.g., Riccia and Marchantia), but in mosses they germinate into filamentous protonema from which are produced buds that give rise to a new plant. The protonema represents the juvenile stage of plant body.
Thus, bryophytes have heteromorphic or heterologous alternation of generation.